1. Field of the Invention
The present invention is generally concerned with hydrostatic transmissions and, more particularly, is concerned with a method of charging a hydrostatic transmission with oil such that the oil will not foam within the hydrostatic transmission.
2. Description of the Prior Art
Hydrostatic transmissions and transaxles (HST's) transmit rotary motion from an input shaft to an output shaft by means of a hydraulic pump and hydraulic motor along with various gearing enclosed within a housing filled with oil. The input shaft is connected, for example, to an engine, while the output shaft can be connected, for example, to the driving wheels of a vehicle such as a lawn and garden tractor. HST's are well known in the art and are manufactured in various configurations. One such HST having a hollow-bodied bellows within an oil reservoir defined by the HST housing, is disclosed in U.S. Pat. No. 4,987,796 entitled, "Internal Reservoir-Defining Body For Permitting Oil Expansion Within A Hydrostatic Transmission Housing," by Roland L. von Kaler filed Jun. 12, 1989, incorporated herein by reference.
An HST cannot function without oil. The oil, however, can be a source of problems working against the efficient operation and long-term reliability of the HST. This is because most oils contain a significant amount of dissolved or entrained air, which is usually on the order of about 10% air per volume of oil. This entrained air is what causes a multitude of problems within the HST.
One such problem is foaming. Foaming occurs as a result of entrained air forming excessive bubbles as the oil is agitated by the pump, motor, gears, and other HST components. This is detrimental to HST function by severely reducing the life and efficiency of the HST components. In addition, foaming causes excess noise during operation.
Another problem with air entrained oil concerns the compressibility of the oil. HST's, like all hydraulic systems, rely on the incompressibility of oil to transmit motion to the various components. Entrained air allows the oil to compress which reduces efficiency.
As a further consequence of air entrained oil, the effective operating temperature of the oil is reduced so that thermal breakdown occurs at a lower temperature. Still further, air entrained oil promotes moisture retention leading to quicker oil breakdown. When the oil breaks down it cannot properly function as intended, causing deleterious effects to the HST.
Deaerated oil has been used in environments such as compressors. Furthermore, several prior art devices and methods have been devised in order to remove entrained air from oils and greases utilizing methods such as vacuum or heat treating, but not in conjunction with a method whereby the deaerated oil is charged into an HST such that further air entrainment will not occur and the air currently within the HST housing is displaced.